The design of heat exchange elements for rotary regenerative heat exchangers is a matter of great difficulty. On the one hand, high heat exchange between the solid element and the fluid medium is desired, and on the other, low pressure drop as the medium flows past the element is just as important, and conventionally advantage in heat exchange efficiency or in lack of pressure drop has been purchased at the cost of accepting less than ideal performance in the other respect.
In rotary regenerative air preheaters, the present standard practice is to increase heat-exchange efficiency by assembling a stack of elements between which the medium is to flow in which the elements are corrugated, with the corrugations of alternate elements running at equal but opposite angles to the direction of flow of the medium.
An example of this is seen in GBPS No. 1000496, which also suggests (FIG. 5) that an alternative conformation for each element would be a zig-zag.
The same idea is expressed, though in a form more efficient from the point of view both of pressure loss and of heat exchange, in GBPS No. 1439674, which was unpublished at the priority date of the present specification. This shows zig-zag corrugated elements sandwiching between them elements having a deeper and less frequent corrugation, with the channels formed by this second type of element running parallel to the direction of flow of the medium.
The sandwiching of elements of one type of conformaton between those of another type to build up a complex but efficient system of flow channels is in itself generally known and an arrangement described below with reference to FIG. 1 has been used for some years in rotary regenerative heat exchangers. This has straight-corrugated elements respectively oppositely angled to the direction of flow of the medium sandwiching an element of deeper and less frequent corrugations which extend along the direction of flow.